This invention relates to the calibration of spectroradiometric instruments and, more particularly, to the calibration of a spectroradiometric standard light source by monitoring one or more photodetector currents outputted by one or more spectrally filtered photodetectors viewing separate spectral bands of radiation outputted by the light source.
A calibration of polychromatic light sources ranging in spectral content from the near ultraviolet with wavelength of approximately 200 nanometers to short wave infrared radiation of wavelength of approximately 2500 nanometers is important for numerous applications including the use of light sources in industry and in laboratories, as well as on spacecraft, by way of example. The term light, as used herein, is understood to include radiation throughout the above-noted spectral range of interest. Space-borne applications for source calibration introduce constraints and difficulties for the calibration process which generally are not found in calibration situations in land-based laboratory calibration of light sources.
Absolute spectroradiometric calibration for a space-borne imaging radiometer usually requires that the calibration be performed at power levels different by several orders of magnitude from that of available standards. This requires precise dynamic range for a standard source capable of outputting radiation at different power levels. One method of altering the intensity of the radiant energy received from a light source is to vary the distance between the source and a receiver of the radiation. However, this procedure assumes an idealized inverse-square law for establishing the intensity of the received radiation. In practice, there are deviations from the inverse-square relationship because of the geometry of the source as well as other factors in the laboratory environment. A further disadvantage in the use of the variation of distance to control power level is the need for a significant increase in the amount of space available for conducting the calibration. Thus, to some extent, the use of a varying distance may be impractical and, furthermore, there may be insufficient precision for calibration when a variable intensity is required.
A further aspect in the attainment of a reliable and stable source of light is the fact that the reference source is operated with a preset amount of current and voltage applied for outputting a reference intensity of radiation. This is based on the presumption that the reference intensity of radiation, attained previously during a calibration procedure with the preset amount of current and voltage, is still valid for later uses of the light source. However, there is an unpredictable drift with time so that, at later usage of the light source, the intensity and spectral distribution may be different, even though the preset amount of current and voltage is being applied for activating the light source. Thus, reliance solely on the values of input electric current and voltage to the light source does not assure adequate precision of calibration of spectroradiometric instruments where high accuracy is required.
A further problem arises, particularly in the spacecraft environment, wherein the light source is operated in a vacuum environment. The light source produces heat during operation of the source, the heat being removed by convection with air currents circulating past the light source during operation of the light source in an air environment. However, in the vacuum environment of the spacecraft, there is no air current for cooling the light source and, as a result, the temperature of the light source rises to a higher value than that present during operation in the air environment. The change in temperature alters the intensity and spectral distribution of light emitted by the source. Thus, a light source calibrated in an air environment in a laboratory may have a different radiation characteristic when operating in vacuum.